Low Blood Pressure and Fainting May Occur When
Viagra and Alpha-blocking Agents Used Together

Source: USA Today

Credit: USA Today story by Rita Rubin

Viagra (sildenafil), taken within hours of the drug Cardura (doxazosin), results in an undesirable hypotensive effect in some patients, according to recent data released by Pfizer. This finding has resulted in addition of a new precaution to the package insert of Viagra. The precaution is of less concern than a "warning," which would herald a more serious interaction, e.g., that seen with nitrates.

According to the revised package insert:

"When the alpha blocker doxazosin (4 mg) and VIAGRA (25 mg) were administered simultaneously to patients with benign prostatic hyperplasia (BPH), mean additional reductions of supine blood pressure of 7 mmHg systolic and 7 mmHg diastolic were observed. When higher doses of VIAGRA and doxazosin (4 mg) were administered simultaneously, there were infrequent reports of patients who experienced symptomatic postural hypotension within 1 to 4 hours of dosing. Simultaneous administration of VIAGRA to patients taking alpha-blocker therapy may lead to symptomatic hypotension in some patients. Therefore, VIAGRA doses above 25 mg should not be taken within 4 hours of taking an alpha-blocker."

This precaution is important because many men on alpha-blocking agents for BPH are also taking the drug Viagra for erectile dysfunction. Moreover, both drugs are often taken in the evening hours. Thus, this interaction is potentially a very common one.

Other alpha-blocking agents in widespread usage today include FloMax (tamsulosin), Hytrin (terazosin), and Minipres (prazosin), with a new entry (alfuzosin) expected in the U.S. in 2003. Although Pfizer issued a general precaution for the entire alpha-blocker class, data are currently available only for Cardura (doxazosin), which was actually used as an anti-hypertensive agent before being used for BPH. The newer alpha-blocking agents, tamsulosin and alfuzosin, have no indication for treatment of hypertension and thus may be less likely to cause this interaction than the other agents.

Complete Viagra package insert (revised 9/02) is posted here:

http://www.viagra.com/professional/pi/proPackInsert.asp#6